The invention relates to a picture display device comprising a display tube having a display screen and an electron gun facing said screen and having a cathode centred along an electron-optical axis and a plurality of electrodes which oily constitute a beam-shaping part for generating an electron beam, said gun further comprising a tubular structure having an outer surface and an inner surface on which a helical resistance structure of a material having a high electrical resistance constituting a focusing lens is provided.
A focusing lens constituted by a helical high-ohmic resistance structure for use in display tubes in order to obtain a low spherical aberration is known from literature.
However, problems occur if a dynamic focusing signal is to be applied to this type of focusing lens. Correction by means of applying a dynamic focusing voltage may be necessary, for example, in the case of large deflection angles so as to keep the electron beam in focus throughout the screen (a different focusing voltage is required in the corners than the centre of the screen). The high resistance of the helical lens structure causes particular problems with regard to the dynamic focus if the frequency of the focusing signal exceeds 16 kHz. This is a result of the large intrinsic RC time of the resistance layer, even at those areas where the layer does not constitute a helical structure, but is homogeneous.